In recent years, in connection with miniaturization of devices accompanying higher integration of semiconductor circuits, a demand for quality of a silicon single crystal produced by Czochralski method (CZ method) which is used as a substrate thereof has been increasing. In particular, there exist defects introduced during growth of a single crystal, called grown-in defects such as FPD, LSTD and COP, which degrade oxide dielectric breakdown voltage characteristic and device characteristics. It has been considered that it is important to reduce a density and a size thereof.
For explanation of these defects, there will be given first general knowledge of factors determining a concentration of void-type point defects called vacancy (hereinafter occasionally abbreviated as “V”) and a concentration of interstitial-type silicon point defects called interstitial silicon (hereinafter occasionally abbreviated as “I”), respectively, which are introduced into a silicon single crystal.
In a silicon single crystal, V region refers to a region which contains a large amount of vacancies, i.e., depressions, holes, or the like generated due to shortage of silicon atoms; and I region refers to a region which contains a large amount of dislocations and clusters of excess silicon atoms generated due to existence of excess silicon atoms. Further, between the V region and the I region, there exists a neutral (hereinafter occasionally abbreviated as “N”) region which contains no (or little) excess and shortage of the atoms. It has become clear that the above-mentioned grown-in defects (such as FPDs, LSTDs and COPs) are generated due to agglomeration of point defects when V or I is present in a supersaturated state, but the point defects do not agglomerate so long as it is not saturated even when there is a little unevenness of atoms, and therefore do not exist as a grown-in defect.
FIG. 7 shows a growth rate of a single crystal and a defect distribution thereof. It has been confirmed that concentrations of the aforementioned both point defects depend on the relation between a pulling rate (growth rate) of the crystal in CZ method and a temperature gradient G near a solid-liquid interface in the crystal, and that another type of defect called OSF (oxidation induced stacking fault) is distributed in a ring shape (hereinafter occasionally referred to as OSF ring) near a boundary between the V region and the I region when the cross section perpendicular to the axis of crystal growth is observed.
When the growth rate is varied from a high speed to a low speed in the direction of the crystal growth axis by use of a CZ pulling apparatus with a general furnace structure (hot zone: HZ) having a large temperature gradient G near a solid-liquid interface of the crystal, these defects introduced during the crystal growth exist as in a distribution diagram of defects shown in FIG. 7.
These defects introduced during the crystal growth can be classified as follows. When the growth rate is relatively high, for example, about 0.6 mm/min or higher, grown-in defects such as FPDs, LSTDs and COPs which are considered to be generated due to voids consisting of agglomerated vacancy-type point defects are present at high density over the entire radial cross section of the crystal. The region where these defects are present is V region (See FIG. 7, line (A)).
When the growth rate is not higher than 0.6 mm/min, the OSF ring is generated from a circumferential portion of the crystal with decrease of the growth rate, and defects (huge dislocation clusters) of L/D (Large Dislocation, abbreviation of interstitial dislocation loop, LSEPD, LFPD or the like) which are considered to be generated due to dislocation loop consisting of agglomeration of interstitial silicons are present at low density outside the ring. The region where these defects are present is I region (occasionally referred to as L/D region). Furthermore, when the growth rate is decreased to about 0.4 mm/min or less, the OSF ring shrinks to the center of a wafer and disappears, so that I region spreads over the entire plane of the wafer (See FIG. 7, line (C)).
As shown in FIG. 7, there has been recently found existence of a region between V region and I region, and outside the OSF ring, called N region, where there exists none of the void-originated FPD, LSTD and COP, and the interstitial silicon-originated LSEPD and LFPD. It has been reported that the region is located outside the OSF ring, shows substantially no oxygen precipitation when the single crystal is subjected to heat treatment for oxygen precipitation and the contrast due to precipitates is observed through X-ray observation or the like, and exists on I region side where the interstitial silicon is not rich enough to form LSEPDs and LFPDs (See FIG. 7, line (B)).
Since such an N region is formed obliquely in the direction of the growth axis when the growth rate is lowered in a general method, it existed only in a part of a plane of wafer.
As for this N region, according to the Voronkov theory (V. V. Voronkov, Journal of Crystal Growth, 59 (1982) 625-643), it is proposed that total concentration of point defects is determined by a parameter called V/G which is a ratio of a pulling rate (V) and a temperature gradient (G) in the axis direction at the crystal solid-liquid interface. Considering the above theory, since the pulling rate (the growth rate) is almost constant in a plane and G has a distribution in the plane, for example, only a crystal wherein V region exists at the center and I region exists around it via N region could be obtained at a certain pulling rate.
Recently improvement of the distribution of G in a plane has been attempted. Then, although the N region could exist only obliquely, it has become possible to produce a crystal in which N region spreads over an entire transverse plane (an entire plane of a wafer) at a certain pulling rate, for example, when the crystal is pulled while gradually decreasing the pulling rate (the growth rate). Enlargement of the crystal having N region over the entire plane into a longitudinal direction can be achieved to some extent by pulling the crystal while maintaining the pulling rate at which the N region transversely spreads. Furthermore, by controlling the pulling rate so that V/G value may be persistently constant while considering that G is varied with growth of the crystal and compensating it, it has also become possible to enlarge the crystal having N region over the entire plane into the direction of growth to some extent.
And also recently, as further classification of the N region, it is known that there exist Nv region (a region where a lot of vacancies exist) adjacent to the outside of the OSF ring and Ni region (a region where a lot of interstitial silicons exist) adjacent to I region, and that the amount of precipitated oxygen becomes large in the Nv region when thermal oxidation treatment is carried out, and substantially no oxygen precipitation is generated in the Ni region.
Therefore, as shown in FIG. 8, in a conventional crystal growth in N region, there is a tendency to obtain a silicon wafer occupied by N region over the entire plane by growing a silicon single crystal in N region including together Nv region where the amount of precipitated oxygen is large and Ni region where substantially no oxide precipitation layer is formed, without distinguishing them, or in either Nv region or Ni region.
However, it has been found that there are some cases that oxide-film defects occur remarkably in spite of a single crystal wherein the N region occupies the entire plane, an OSF ring is not generated when thermal oxidation treatment is carried out, and FPD and L/D do not exist over the entire plane as mentioned above. This may be a cause of degrading electrical characteristics such as oxide dielectric breakdown voltage characteristic. Accordingly, the fact that the N region merely occupies the entire plane as in a conventional crystal is not enough, and further improvement of electrical characteristics has been desired. Moreover, since dispersion of gettering capability occurs even if the crystals are grown in the above-described Nv region where the oxygen precipitation is large, yield of devices may not be necessarily improved. Accordingly, further improvement of quality has been desired.